The INPUT_SECTION_ALIGN command affects INPUT_SECTION commands. Instead use the ALIGN(number) command - it aligns the address of the current location counter to the next address that is a multiple of number, where number is a power of 2.

You can also define the buffer in your application C or assembly source to avoid the need for an LDF modification.

When I tried this, replacing INPUT_SECTION_ALIGN(4) with ALIGN(4), the _LARGE_BUF_BEGIN address did change from 0x801c2017 to 0x801c2018, and _LARGE_BUF_END accordingly changed to 0x801c3018. Can you re-check?